Feldspar in type 4–6 ordinary chondrites: Metamorphic processing on the H and LL chondrite parent bodies

Abstract– We have carried out a study of feldspar compositions in a suite of H and LL ordinary chondrites, of petrologic types 4, 5, and 6, in order to examine the process of recrystallization and equilibration of feldspar as the degree of metamorphism increases. In the H chondrites, there is little variation in feldspar compositions among the petrologic types, suggesting that homogenization of chondrule mesostasis, from which feldspar is presumed to have crystallized, occurred before feldspar crystallization began. The LL chondrites we studied are more complex. In Bjurböle (L/LL4), plagioclase in individual relict chondrules has distinct compositions, with a range of An/Ab ratios and low Or contents. This heterogeneity is most likely attributable to original compositional heterogeneity among chondrule mesostases: localized recrystallization of mesostasis must have occurred before diffusional equilibration took place. In Tuxtuac (LL5), the An/Ab ratio of plagioclase is more homogeneous, and plagioclase includes a significant Or component. In addition, we observe what appears to be exsolution of K‐feldspar from albitic host grains. In Saint Séverin (LL6), the An/Ab ratio of plagioclase is homogeneous, but plagioclase compositions show a range of Or contents, corresponding to a patchy distribution of K in individual feldspar grains. The observations in these LL chondrites are difficult to interpret with a simple model of progressive equilibration with increasing petrologic type. We suggest that the current criteria for assigning petrologic types are poorly defined: it is possible that the assigned petrologic types of these chondrites do not correlate with their peak temperatures. We propose that feldspar compositions might record conditions during the heating stage of metamorphism, and that the early stages of metamorphism may have occurred in the presence of fluids, rather than under the dry conditions that are commonly assumed.     

HAVERÖ UREILITE: EVIDENCE FOR RECRYSTALLIZATION AND PARTIAL REDUCTION

Haverö consists of large olivine areas with a pavement structure and single crystals of twinned clinopyroxene. Black veins with sharp boundaries traverse the silicates. They contain graphite, diamond, and kamacite. In olivine a reaction rim is formed around these veins containing Ni-poor metal and showing a lower FeO content than farther away from the vein. The CaO content of olivine and pyroxene, 0.27% and 1.7%, respectively, are higher than in these minerals in normal chondrites. The mole percent Fe + Ca/Fe + Ca + Mg in unchanged olivine and in pyroxene agree with the range of L-chondrites. Metal occurs in three types: a, larger grains in the course of the black veins, they contain 2 to 3% Ni; b, micron-sized grains inside the black veins and its reaction rim; c, medium-sized grains with ～0.7% Ni in olivine The interpretation of these observations is: a material similar to an L-chondrite was reheated and recrystallized (at this time it may have lost its feldspar, metal and troilite by partial melting), Ca was redistributed and partly retained in olivine and pyroxene due to rapid cooling, a late introduction of carbon into veins caused a partial reduction of FeO in olivine and formation of Ni-poor metal     

Kosmochloric Ca‐rich pyroxenes and FeO‐rich olivines (Kool grains) and associated phases in Stardust tracks and chondritic porous interplanetary dust particles: Possible precursors to FeO‐rich type II chondrules in ordinary chondrites

Abstract— Terminal particles and mineral fragments from comet 81P/Wild 2 were studied in 16 aerogel tracks by transmission and secondary electron microscopy. In eight tracks clinopyroxenes with correlated Na₂O and Cr₂O₃ contents as high as 6.0 wt% and 13.0 wt%, respectively, were found. Kosmochloric (Ko) clinopyroxenes were also observed in 4 chondritic interplanetary dust particles (IDPs). The Ko‐clinopyroxenes were often associated with FeO‐rich olivine ± Cr‐rich spinel ± aluminosilicate glass or albitic feldspar, assemblages referred to as Kool grains (Ko = kosmochloric Ca‐rich pyroxene, ol = olivine). Fine‐grained (submicron) Kool fragments have textures suggestive of crystallization from melts while coarse‐grained (>1 μm) Kool fragments are often glass‐free and may have formed by thermal metamorphism in the nebula. Average major and minor element distributions between clinopyroxenes and coexisting FeO‐rich olivines are consistent with these phases forming at or near equilibrium. In glass‐bearing fine‐grained Kool fragments, high concentrations of Na in the clinopyroxenes are inconsistent with existing experimentally determined partition coefficients at equilibrium. We speculate that the availability of Cr in the melt increased the clinopyroxene Na partition coefficient via a coupled substitution thereby enhancing this phase with the kosmochlor component. The high temperature minerals, fine‐grain sizes, bulk compositions and common occurrence in the SD tracks and IDPs support the idea that Kool grains could have been precursors to type II chondrules in ordinary chondrites. These grains, however, have not been observed in these meteorites suggesting that they were destroyed during chondrule formation and recycling or were not present in the nebula at the time and location where meteoritic chondrules formed.     

NIOBIAN RUTILE IN AN APOLLO 14 KREEP FRAGMENT

Niobian rutile was found in a KREEP lithic fragment of basaltic texture. The niobian rutile contains 85.3% TiO₂, 7.1% Nb₂O₅, 2.65% Cr₂O₃, 0.70% ZrO₂, 0.61% SiO₂, 0.82% Al₂O₃ 0.61% FeO, 0.52% CaO, 0.22% V₂O₃ in addition to minor amounts of MnO, MgO, and CeO₂. Rare earth elements were not detected, in contrast with lunar niobian rutile of Marvin (1971). Coexisting minerals in the KREEP fragment are major amounts of plagioclase and orthopyroxene, and minor amounts of olivine, ilmenite, augite, barian K-feldspar, whitlockite, troilite, Ni-Fe, zirkelite and chromite     

CLAST-LADEN MELT-ROCK FRAGMENT IN THE ADAMS COUNTY,COLORADO, H5 CHONDRITE

The Adams County, Colorado, H5 chondrite contains a lithic fragment, 1 cm in size, that is texturally and mineralogically quite different from the chondritic host. It is composed of: a groundmass of fine-grained euhedral to subhedral olivine (3–15 μm) and interstitial glass enclosing larger olivine and pyroxene grains (0.15-0.5 mm; about 15 vol %); an assemblage of enstatite grains (subfragment within) and an assemblage of olivine plus orthopyroxene (a second subfragment); and about 11 vol % grains of mixed troilite and nickel-iron metal. Analyses yielded these results: (i) olivine grains of the fragment groundmass have a compositional range (Fa_12–45) and most grains contain substantial CaO and Cr₂O₃ (～ 0.20 and 0.30 avg. wt%, respectively); interstitial glass has ～ 55 wt% SiO₂; (ii) larger olivine grains of the fragment are similarly high in CaO and Cr₂O₃ and also have a wide FeO/MgO range; one unusual pyroxene is an Mg-rich pigeonite; (iii) the metal is martensite in composition (11–14 wt% Ni); and (iv) major and trace element analyses by INAA indicate an H-group bulk composition for the entire 1 cm lithic fragment. On the basis of its texture and bulk and mineral compositions, the fragment is interpreted to represent unequilibrated H-group material that was partly melted by impact. The Ca- and Cr-enriched groundmass olivine and interstitial glass resulted from rapid crystallization of the chondritic melt. The Ca- and Cr-enriched larger silicate grains, including the enstatite sub-fragment and the pigeonite grain, are residual, unmelted clasts from the target material (this is supported by the presence of similar material in actual H3 chondrites). Further impact brecciation of the clast-laden melt material, and resultant impact-splashing accounts for the presence of the fragment in the H-group Adams County host and documents the coexistence of unequilibrated and equilibrated H-group material as surface regolith on one parent body.     

Petrogenesis of lunar meteorite Northwest Africa 2977: Constraints from in situ microprobe results

Abstract– Northwest Africa (NWA) 2977 is an olivine‐gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. We studied this rock with a series of in situ analytical methods. NWA 2977 consists mainly of olivine and pyroxene with minor plagioclase. It shows evidence of intense shock metamorphism, locally as high as shock‐stage S6. Olivine adjacent to a melt vein has been partially transformed into ringwoodite and Al,Ti‐rich chromite grains have partially transformed into their high‐pressure polymorph (possibly CaTi₂O₄‐structure). Olivine in NWA 2977 contains two types of lithic inclusions. One type is present as Si,Al‐rich melt inclusions that are composed of glass and, in most cases, dendritic pyroxene. The other type is mafic and composed of relatively coarse‐grained augite with accessory chromite, RE‐merrillite, and baddeleyite. Two Si,Al‐rich melt inclusions are heavy rare earth elements (REE) enriched, whereas the mafic inclusion has high REE concentrations and a KREEP‐like pattern. The mafic inclusion could be a relict fragment captured during the ascent of the parent magma of NWA 2977, whereas the Si,Al‐rich inclusions may represent the original NWA 2977 melt. The calculated whole‐rock composition has a KREEP‐like REE pattern, suggesting that NWA 2977 has an affinity to KREEP rocks. Baddeleyite has recorded a young crystallization age of 3123 ± 7 Ma (2σ), which is consistent with results from previous whole‐rock and mineral Sm‐Nd and Rb‐Sr studies. The petrography, mineralogy, trace element geochemistry, and young crystallization age of NWA 2977 support the possibility of pairing between NWA 2977 and the olivine‐gabbro portion of NWA 773.     

Planar deformation features and impact glass in inclusions from the Vredefort Granophyre,South Africa

Abstract— The Vredefort Granophyre represents impact melt that was injected downward into fractures in the floor of the Vredefort impact structure, South Africa. This unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite, feldspathic quartzite, arkose, and granitic material with minor proportions of shale and epidiorite. Two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features. Quartz grains in other inclusions display a vermicular texture, which is reminiscent of checkerboard feldspar. Feldspars range from large, twinned crystals in some inclusions to fine‐grained aggregates that apparently are the product of decomposition of larger primary crystals. In rare inclusions, a mafic mineral, probably biotite or amphibole, has been transformed to very fine‐grained aggregates of secondary phases that include small euhedral crystals of Fe‐rich spinel. These data indicate that inclusions within the Vredefort Granophyre were exposed to shock pressures ranging from <5 to 8–30 GPa. Many of these inclusions contain small, rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals, including K‐feldspar, quartz, augite, low‐Ca pyroxene, and magnetite. The composition of this devitrified glass varies from inclusion to inclusion, but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals. In the case of granitoid inclusions, melt pockets commonly occur at the boundaries between feldspar and quartz grains. In metasedimentary inclusions, some of these melt pockets contain remnants of partially melted feldspar grains. These melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot (650 to 1610 °C) impact melt that crystallized to form the Vredefort Granophyre.     

THE IJOPEGA CHONDRITE: A NEW H6 FALL

The Ijopega (Papua New Guinea) meteorite is a new H6 group chondrite fall which contains olivine (Fa 19.9 mole %), bronzite (Fs 17.8 mole %), plagioclase (An 12.1 Or 6.3 Ab 81.6 mole %), diopside, kamacite, taenite, troilite, chromite and whitlockite. The meteorite is extensively recrystallized and brecciated, and shows evidence of moderate shock deformation. Examination of Fe²⁺ and Mg partitioning between ortho- and clinopyroxene indicates a high equilibration temperature (940° or 880 °C). Chemical analysis shows the meteorite to be rich in S, containing about twice the average H-group abundance. Trace elements, including REE, are in accord with established H-group chondrite abundances.     

The Yarle Lakes 001 meteorite: A H5 chondrite from South Australia

Abstract— The Yarle Lakes 001 meteorite was a single stone of 913 g found approximately 20 km north of Watson, South Australia, in 1990 October. It consists of olivine (Fa_{18.7 ± 0.4}, n = 30), low-Ca pyroxene (Fs_{16.6 ± 0.2} Wo _{12 + 0.4}, n = 15). feldspar, high-Ca pyroxene, metallic Fe-Ni and troilite. Based on texture and mineral chemistry, Yarle Lakes 001 is classified as a H5 chondrite of shock stage S3.     

Complexly zoned chromium‐aluminum spinel found in situ in the Allende meteorite

Abstract— In addition to the Mg‐, Al‐, ¹⁶O‐rich spinels that are known to occur in refractory inclusions, the Murchison meteorite contains Cr‐rich, ¹⁶O‐poor spinels, most of whose sources are unknown because they are rarely found in situ. Here we report the in situ occurrence in Allende of Cr‐rich spinels, found in 13 chondrules and 4 “olivine‐rich objects”. The Allende spinels exhibit major and minor element contents, isotopic compositions, and zoning of Cr₂O₃ contents like those of the Cr‐spinels from Murchison. Some chondrules contain patchy‐zoned spinel (Simon et al., 1994), which suggests that such grains did not form by sintering but perhaps by formation of overgrowths on relic grains. Unlike the olivine‐rich objects, phases in all three chondrules that were analyzed by ion microprobe have uniform, near‐normal O‐isotopic compositions. One olivine‐rich object, ALSP1, has a huge (1 mm) fragment of chevron‐zoned spinel. This spinel has near‐normal O‐isotopic compositions that are quite distinct from those of adjacent forsteritic olivine, which are relatively ¹⁶O‐rich and plot on the calcium‐aluminum‐inclusion (CAI) line, like some isolated forsterite grains found in Allende. The spinel and olivine in this object are therefore not genetically related to each other. Another olivine‐rich object, ALSP11A, contains a rectangular, 150 ×s 100 μm, homogeneous spinel grain with 50 wt% Cr₂O₃ and 23 wt% FeO in a vuggy aggregate of finer‐grained (5–90 μm), FeO‐rich (Fo_47–55) olivine. The magnesian core of one olivine grain has a somewhat ¹⁶O‐rich isotopic composition like that of the large spinel, whereas the FeO‐rich olivine is relatively ¹⁶O‐poor. The composition of the spinel in ALSP11A plots on the CAI line, the first Cr‐rich spinel found to do so. Chevron‐zoned spinel has not been observed in chondrules, and it is unlikely that either ALSP1 or ALSP11A were ever molten. Calculations show that a spinel with the composition of that in ALSP1 can condense at 1780 K at a P^tot of 10^?3 atm and a dust/gas ratio of 100 relative to solar. The Cr‐rich spinel in ALSP11A could condense at ～1420 K, but this would require a dust/gas enrichment of 1000 relative to solar. The data presented here confirm that, as in Murchison, the coarse Cr‐rich spinels in Allende are relatively ¹⁶O‐depleted and are isotopically distinct from the ¹⁶O‐enriched MgAl₂O₄ from CAIs. Sample ALSP11A may represent a third population, one that is Cr‐rich and plots on the CAI line. That the O‐isotopic composition of ALSP1 is like those of Cr‐rich spinels from chondrules indicates that O‐isotopic compositions cannot be used to distinguish whether grains from such unequilibrated objects are condensates or are fragments from a previous generation of chondrules.     

Mineralogy and petrology of amoeboid olivine inclusions in CO3 chondrites: Relationship to parent‐body aqueous alteration

Abstract— Petrographic and mineralogic studies of amoeboid olivine inclusions (AOIs) in CO3 carbonaceous chondrites reveal that they are sensitive indicators of parent‐body aqueous and thermal alteration. As the petrologic subtype increases from 3.0 to 3.8, forsteritic olivine (Fa_0–1) is systematically converted into ferroan olivine (Fa_60–75). We infer that the Fe, Si and O entered the assemblage along grain boundaries, forming ferroan olivine that filled fractures and voids. As temperatures increased, Fe⁺² from the new olivine exchanged with Mg⁺² from the original AOI to form diffusive haloes around low‐FeO cores. Cations of Mn⁺², Ca⁺² and Cr⁺³ were also mobilized. The systematic changes in AOI textures and olivine compositional distributions can be used to refine the classification of CO3 chondrites into subtypes. In subtype 3.0, olivine occurs as small forsterite grains (Fa_0–1), free of ferroan olivine. In petrologic subtype 3.2, narrow veins of FeO‐rich olivine have formed at forsterite grain boundaries. With increasing alteration, these veins thicken to form zones of ferroan olivine at the outside AOI margin and within the AOI interior. By subtype 3.7, there is a fairly broad olivine compositional distribution in the range Fa_63–70, and by subtype 3.8, no forsterite remains and the high‐Fa peak has narrowed, Fa_64–67. Even at this stage, there is incomplete equilibration in the chondrite as a whole (e.g., data for coarse olivine grains in Isna (CO3.8) chondrules and lithic clasts show a peak at Fa₃₉). We infer that the mineral changes in AOI identified in the low petrologic types required aqueous or hydrothermal fluids whereas those in subtypes ?3.3 largely reflect diffusive exchange within and between mineral grains without the aid of fluids.     

Evidence in CO3.0 chondrules for a drift in the O isotopic composition of the solar nebula

Abstract— Several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher Δ¹⁷O (=δ¹⁷O ? 0.52 × δ¹⁸O) values than those recorded in unaltered chondrules. Other recent studies have shown that, in sets of chondrules from carbonaceous chondrites, Δ¹⁷O tends to increase as the FeO contents of the silicates increase. We report a comparison of the O isotopic composition of olivine phenocrysts in low‐FeO (≤Fa₁) type I and high‐FeO (≥Fa₁₅) type II porphyritic chondrules in the highly primitive CO3.0 chondrite Yamato‐81020. In agreement with a similar study of chondrules in CO3.0 ALH A77307 by Jones et al. (2000), Δ¹⁷O tends to increase with increasing FeO. We find that Δ¹⁷O values are resolved (but only marginally) between the two sets of olivine phenocrysts. In two of the high‐FeO chondrules, the difference between Δ¹⁷O of the late‐formed, high‐FeO phenocryst olivine and those in the low‐FeO cores of relict grains is well‐resolved (although one of the relicts is interpreted to be a partly melted amoeboid olivine inclusion by Yurimoto and Wasson [2002]). It appears that, during much of the chondrule‐forming period, there was a small upward drift in the Δ¹⁷O of nebular solids and that relict cores preserve the record of a different (and earlier) nebular environment.     

Refractory inclusions in the unique carbonaceous chondrite Acfer 094

Abstract– Acfer 094 is an unshocked, nearly unaltered carbonaceous chondrite with an unusual suite of refractory inclusions. The refractory inclusions in a newly prepared thin section and a small aliquot of disaggregated material were studied to compare the population with previous work, and to report new or unusual inclusion types. A total of 289 Ca‐, Al‐rich inclusions in the thin section and 67 among the disaggregated material, having a total of 31 different mineral assemblages, were found. Inclusions are largely free of secondary alteration products, and are typically ≤200 μm across. The most common are gehlenitic melilite+spinel±perovskite, spinel+perovskite, and spinel with a thin, silicate rim, typically melilite±diopside. Such rims and (thicker) mantles are very common among Acfer 094 inclusions, and they exhibit a variety of zoning patterns with respect to åkermanite and FeO contents. In the thin section, about 13% of the inclusions contain hibonite and approximately 5% are grossite‐bearing; in the disaggregated material, the percentages are 14 and 9, respectively, comparable to previous work. Among the unusual inclusions are a fine‐grained, porous, Ti‐rich hibonite+spinel+perovskite+melilite inclusion with a compact, coarse, Ti‐poor hibonite+spinel+melilite clast; two inclusions in which hibonite has reacted to form grossite; two inclusions with FeO‐rich spinel; and a small object consisting of fassaite enclosing euhedral spinel, the first fragment of a Type B inclusion reported from Acfer 094. Inclusions similar to those found in CM or CV chondrites are rare; Acfer 094 contains a distinctive population of inclusions. The population, dominated by small, melilite‐bearing inclusions, is most similar to that of CO chondrites. A distinguishing feature is that in Acfer 094, almost every phase in almost every refractory inclusion contains 0.5–1.5 wt% FeO. A lack of diffusion gradients and the pristinity of the matrix imply that the inclusions experienced prolonged exposure to FeO‐bearing fluid prior to accretion into the Acfer 094 parent body. There are no known nebular conditions under which the refractory phases found in the present samples could acquire FeO enrichments to the observed levels. The most likely setting is therefore in an earlier, FeO‐rich parent body. The inclusions were ejected from this parent body, mixed with typical CAIs, chondrules, amoeboid olivine aggregates, and amorphous material, and incorporated into the Acfer 094 parent body.     

Mineralogy,petrology, and oxygen isotopic compositions of chondritic and achondritic lithologies in the anomalous CB carbonaceous chondrites Sierra Gorda 013 and Fountain Hills

The CB (Bencubbin-like) metal-rich carbonaceous chondrites are subdivided into the CB_a and CB_b subgroups. The CB_a chondrites are composed predominantly of ~cm-sized skeletal olivine chondrules and unzoned Fe,Ni-metal ± troilite nodules. The CB_b chondrites are finer grained than the CB_as and consist of chemically zoned and unzoned Fe,Ni-metal grains, Fe,Ni-metal ± troilite nodules, cryptocrystalline and skeletal olivine chondrules, and rare refractory inclusions. Both subgroups contain exceptionally rare porphyritic chondrules and no interchondrule fine-grained matrix, and are interpreted as the products of a gas–melt impact plume formed by a high-velocity collision between differentiated planetesimals about 4562 Ma. The anomalous metal-rich carbonaceous chondrites, Fountain Hills and Sierra Gorda 013 (SG 013), have bulk oxygen isotopic compositions similar to those of other CBs but contain coarse-grained igneous clasts/porphyritic chondrule-like objects composed of olivine, low-Ca-pyroxene, and minor plagioclase and high-Ca pyroxene as well as barred olivine and skeletal olivine chondrules. Cryptocrystalline chondrules, zoned Fe,Ni-metal grains, and interchondrule fine-grained matrix are absent. In SG 013, Fe,Ni-metal (~80 vol%) occurs as several mm-sized nodules; magnesiochromite (Mg-chromite) is accessory; daubréelite and schreibersite are minor; troilite is absent. In Fountain Hills, Fe,Ni-metal (~25 vol%) is dispersed between chondrules and silicate clasts; chromite and sulfides are absent. In addition to a dominant chondritic lithology, SG 013 contains a chondrule-free lithology composed of Fe,Ni-metal nodules (~25 vol%), coarse-grained olivine and low-Ca pyroxene, interstitial high-Ca pyroxene and anorthitic plagioclase, and Mg-chromite. Here, we report on oxygen isotopic compositions of olivine, low-Ca pyroxene, and ±Mg-chromite in Fountain Hills and both lithologies of SG 013 measured in situ using an ion microprobe. Oxygen isotope compositions of olivine, low-Ca pyroxene, and Mg-chromite in these meteorites are similar to those of magnesian non-porphyritic chondrules in CB_a and CB_b chondrites: on a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), they plot close to a slope-1 (primitive chondrule mineral) line and have a very narrow range of Δ¹⁷O (=δ¹⁷O–0.52 × δ¹⁸O) values, −2.5 ± 0.9‰ (avr ± 2SD). No isotopically distinct relict grains have been identified in porphyritic chondrule-like objects. We suggest that magnesian non-porphyritic (barred olivine, skeletal olivine, cryptocrystalline) chondrules in the CB_as, CB_bs, and porphyritic chondrule-like objects in SG 013 and Fountain Hills formed in different zones of the CB impact plume characterized by variable pressure, temperature, cooling rates, and redox conditions. The achondritic lithology in SG 013 represents fragments of one of the colliding bodies and therefore one of the CB chondrule precursors. Fountain Hills was subsequently modified by impact melting; Fe,Ni-metal and sulfides were partially lost during this process.     

An amoeboid olivine inclusion (AOI) in CK3 NWA 1559, comparison to AOIs in CV3 Allende,and the origin of AOIs in CK and CV chondrites

An amoeboid olivine inclusion in CK3 NWA 1559 (0.54 × 1.3 mm) consists of a diopside‐rich interior (approximately 35 vol%) and an olivine‐rich rim (approximately 65 vol%). It is the first AOI to be described in CK chondrites; the apparent paucity of these inclusions is due to extensive parent‐body recrystallization. The AOI interior contains irregular 3–15 μm‐sized Al‐bearing diopside grains (approximately 70 vol%), 2–20 μm‐sized pores (approximately 30 vol%), and traces of approximately 2 μm plagioclase grains. The 75–160 μm‐thick rim contains 20–130 μm‐sized ferroan olivine grains, some with 120º triple junctions. A few coarse (25–50 μm‐sized) patches of plagioclase with 2–18 μm‐thick diopside rinds occur in several places just beneath the rim. The occurrence of olivine rims around AOI‐1 and around many AOIs in CV3 Allende suggests that CK and CV AOIs formed by the acquisition of porous forsteritic rims around fine‐grained, rimless CAIs that consisted of diopside, anorthite, melilite, and spinel. Individual AOIs in carbonaceous chondrites may have formed after transient heating events melted their olivine rims as well as portions of the underlying interiors. In AOI‐1, coarse plagioclase grains with diopside rinds crystallized immediately below the olivine rim. Secondary parent‐body alteration transformed forsterite in the rims of CV and CK AOIs into more‐ferroan olivine. Some of the abundant pores in the interior of AOI‐1 may have formed during aqueous alteration after fine‐grained melilite and anorthite were leached out. Chondrite groups with large chondrules tend to have large AOIs. AOIs that formed in dust‐rich nebular regions (where CV and CK chondrites later accreted) tend to be larger than AOIs from less‐dusty regions.     

Origin of fayalitic olivine rims and lath-shaped matrix olivine in the CV3 chondrite Allende and its dark inclusions

Abstract— We have studied an Allende dark inclusion by optical microscopy, scanning electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated olivines and matrix, which, as in the Allende host, is mainly composed of 5–20 μm long lath-shaped fayalitic grains with a narrow compositional range (Fa_{42 ± 2}) and nepheline. Olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5–10 μm wide fayalitic rims (Fa_{39 ± 2}) and 100–1000 μm wide translucent zones, which consist of 5–20 μm long lath-shaped fayalitic grains (Fa_{41 ± 1}) intergrown with nepheline. These fayalitic olivines, like those in the matrix of the dark inclusion, contain 10–20 nm sized inclusions of chromite, hercynite, and Fe-Ni sulfides. The fayalitic rims around remnant olivines are texturally and compositionally identical to those in Allende host, suggesting that they have similar origins. Chondrules are surrounded by opaque rims consisting of tiny lath-shaped fayalitic olivines (<1–3 μm long) intergrown with nepheline. As in the Allende host, fayalitic olivine veins may crosscut altered chondrules, fine-grained chondrule rims and extend into the matrix, indicating that alteration occurred after accretion. We infer that fayalitic olivine rims and lath-shaped fayalites in Allende and its dark inclusions formed from phyllosilicate intermediate phases. This explanation accounts for (1) the similarity of the replacement textures observed in the dark inclusion and Allende host to aqueous alteration textures in CM chondrites; (2) the anomalously high abundances of Al and Cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath-shaped fayalites in the Allende dark inclusion, which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates and then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete dehydration. The absence of low-Ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in Allende is consistent with the lower resistance of low-Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydro-thermal processing of Allende probably also accounts for the low abundance of planetary noble gases and interstellar grains, and the formation of nepheline, sodalite, salite-hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite in chondrules and Ca,Al-rich inclusions.     

The Miller Range 090340 and 090206 meteorites: Identification of new brachinite‐like achondrites with implications for the diversity and petrogenesis of the brachinite clan

Miller Range (MIL) 090340 and MIL 090206 are olivine‐rich achondrites originally classified as ureilites. We investigate their petrography, mineral compositions, olivine Cr valences, equilibration temperatures, and (for MIL 090340) oxygen isotope compositions, and compare them with ureilites and other olivine‐rich achondrites. We conclude that they are brachinite‐like achondrites that provide new insights into the petrogenesis of brachinite clan meteorites. MIL 090340,6 has a granoblastic texture and consists of ~97 modal % by area olivine (Fo = molar Mg/[Mg+Fe] = 71.3 ± 0.6). It also contains minor to trace augite, chromite, chlorapatite, orthopyroxene, metal, troilite, and terrestrial Fe‐oxides. Approximately 80% by area of MIL 090206,5 has a granoblastic texture of olivine (Fo 72.3 ± 0.1) plus minor augite and chromite, similar to MIL 090340 but also containing minor plagioclase. The rest of the section consists of a single crystal of orthopyroxene (~11 × 3 mm), poikilitically enclosing rounded grains of olivine (Fo = 76.1 ± 0.6), augite, chromite, metal, and sulfide. Equilibration temperatures for MIL 090340 and MIL 090206, calculated from olivine‐spinel, olivine‐augite, and two‐pyroxene thermometry range from ~800 to 930 °C. In both samples, symplectic intergrowths of Ca‐poor orthopyroxene + opaque phases (Fe‐oxides, sulfide, metal) occur as rims on and veins/patches within olivine. Before terrestrial weathering, the opaques were probably mostly sulfide, with minor metal. All petrologic properties of MIL 090340 and MIL 090206 are consistent with those of brachinite clan meteorites, and largely distinct from those of ureilites. Oxygen isotope compositions of olivine in MIL 090340 (δ¹⁸O = 5.08 ± 0.30‰, δ¹⁷O = 2.44 ± 0.21‰, and Δ¹⁷O = ?0.20 ± 0.12‰) are also within the range of brachinite clan meteorites, and well distinguished from ureilites. Olivine Cr valences in MIL 090340 and the granoblastic area of MIL 090206 are 2.57 ± 0.06 and 2.59 ± 0.07, respectively, similar to those of three brachinites also analyzed here (Brachina, Hughes 026, Nova 003). They are higher than those of olivine in ureilites, even those containing chromite. The valence systematics of MIL 090340, MIL 090206, and the three analyzed brachinites (lower Fo = more oxidized Cr) are consistent with previous evidence that brachinite‐like parent bodies were inherently more oxidized than the ureilite parent body. The symplectic orthopyroxene + sulfide/metal assemblages in MIL 090340, MIL 090206, and many brachinite clan meteorites have superficial similarities to characteristic “reduction rims” in ureilites. However, they differ significantly in detail. They likely formed by reaction of olivine with S‐rich fluids, with only minor reduction. MIL 090340 and the granoblastic area of MIL 090206 are similar in modal mineralogy and texture to most brachinites, but have higher Fo values typical of brachinite‐like achondrites. The poikilitic pyroxene area of MIL 090206 is more typical of brachinite‐like achondrites. The majority of their properties suggest that MIL 090340 and MIL 090206 are residues of low‐degree partial melting. The poikilitic area of MIL 090206 could be a result of limited melt migration, with trapping and recrystallization of a small volume of melt in the residual matrix. These two samples are so similar in mineral compositions, Cr valence, and cosmic ray exposure ages that they could be derived from the same lithologic unit on a common parent body.     

Isolated olivines in the Yamato 82042 CM2 chondrite: The tracing of major condensation events in the solar nebula

Abstract— Yamato 82042 is an unusual CM2 chondrite consisting mainly of phyllosilicates, a few olivines and carbonates, very minor sulphides and trace metal. Olivine occurs: (1) as isolated grains dispersed in the phyllosilicate matrix, (2) as constituents of mineral aggregates or accretionary fragments associated with abundant phyllosilicates and minor sulphides, and (3) as objects which resemble barred olivine chondrules also associated with phyllosilicates. Olivine, from all occurrences, ranges in composition from 0.26 to 22.6 weight % FeO, but generally contains less than 1.25 wt.% FeO. Minor element contents, particularly Ca, Al, and Cr, are relatively high and are generally correlated, as reported for olivines in other carbonaceous chondrites. However, we report here uncorrected trends for the same minor elements which occur in distinct areas (volumes) within the same olivines. These compositional trends may be due to condensation of olivine from a vapor of non-solar composition and partial mobilization of Ca during later annealing. If this is the case, the data may be used to trace changes in the Ca/Al ratio of the parent medium during the formation of these olivines, provided that it is possible to distinguish the effects of any post-formation annealing which could have redistributed the minor elements. Some isolated olivines show distinctive minor element zoning which severely limits the possibility of any post-formation redistribution of these elements. Accordingly, these isolated olivines indeed retain evidence of early condensation processes in the solar nebula, though non-classic conditions are implied for their formation.     

Zoning patterns of Fe and V in spinel from a type B Ca‐Al‐rich inclusion: Constraints on subsolidus thermal history

Abstract We obtained two‐dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS‐34 with a 4–5 μm resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS‐34 contained V‐rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ?8 μm and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600–700°C, this leads to a time scale for calcium‐aluminum‐rich inclusion (CAI) alteration in the range of decades to centuries.     

Petrography of refractory inclusions in CM2.6 QUE 97990 and the origin of melilite‐free spinel inclusions in CM chondrites

Abstract— Queen Alexandra Range (QUE) 97990 (CM2.6) is among the least‐altered CM chondrites known. It contains 1.8 vol% refractory inclusions; 40 were studied from a single thin section. Inclusion varieties include simple, banded and nodular structures as well as simple and complex distended objects. The inclusions range in mean size from 30 to 530 μm and average 130 ± 90 μm. Many inclusions contain 25 ± 15 vol% phyllosilicate (predominantly Mg‐Fe serpentine); several contain small grains of perovskite. In addition to phyllosilicate, the most abundant inclusions in QUE 97990 consist mainly of spinel‐pyroxene (35%), followed by spinel (20%), spinel‐pyroxene‐olivine (18%), pyroxene (12%), pyroxene‐olivine (8%) and hibonite ± spinel (8%). Four pyroxene phases occur: diopside, Al‐rich diopside (with ≥ 8.0 wt% Al₂O₃), Al‐Ti diopside (i.e., fassaite), and (in two inclusions) enstatite. No inclusions contain melilite. Aqueous alteration of refractory inclusions transforms some phases (particularly melilite) into phyllosilicate; some inclusions broke apart during alteration. Melilite‐free, phyllosilicate‐bearing, spinel inclusions probably formed from pristine, phyllosilicate‐free inclusions containing both melilite and spinel. Sixty‐five percent of the refractory inclusions in QUE 97990 appear to be largely intact; the major exception is the group of spinel inclusions, all of which are fragments. Whereas QUE 97990 contains about 50 largely intact refractory inclusions/cm², estimates from literature data imply that more‐altered CM chondrites have lower modal abundances (and lower number densities) of refractory inclusions: Mighei (CM ? 2.3) contains roughly 0.3–0.6 vol% inclusions (?10 largely intact inclusions/cm²); Cold Bokkeveld (CM2.2) contains ?0.01 vol% inclusions (on the order of 6 largely intact inclusions/cm²).